Using dependency injection to improve testability

get_tax.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'rspec', '~> 3.10'
  gem 'activesupport', require: 'active_support/all'
end

module Calc
  def self.get_tax(amount, time: Time.now)
    tax = 7
    tax = tax + 10 if amount.to_f >= 1000
    if time.on_weekday?
      tax = 5 if time.hour >= 9 && time.hour <= 18
    end
    tax
  end
end

require 'rspec/autorun'

RSpec.describe Calc do
  describe '.get_tax' do
    context 'when the amount is greater than or equal to 1000' do
      context 'and now is a weekday but is not a business hour' do
        it 'returns 17' do
          time = double(on_weekday?: true, hour: 8)

          tax = Calc.get_tax(1001, time: time)

          expect(tax).to be == 17
        end
      end
    end
  end
end

Refactoring Calc.get_tax.

module Calc
  def self.get_tax(amount, time: Time.now)
    tax = amount.to_i >= 1000 ? 17 : 7

    return tax unless time.on_weekday?

    time.hour >= 9 && time.hour <= 18 ? 5 : tax
  end
end